Project Summary Exocytosis is mediated by transport, docking and fusion of secretory vesicles from trans-Golgi network and endosomes to the plasma membrane. As exocytosis is centrally involved, and tightly controlled, in a wide range of physiological processes from hormone release to embryogenesis, it is important to understand the molecular mechanisms that regulate exocytosis. Our research focuses on two separate but connected modules in the exocytosis pathway, the Rab GTPases and the exocyst complex, both of which are evolutionarily conserved from yeast to humans. In this proposal, we will first investigate the molecular mechanisms that control the activation of Rabin8, which is the guanine nucleotide exchange factor (GEF) for Rab8, a major regulator of exocytosis in mammalian cells. We will examine the role of phosphatidylinositides and an upstream Rab protein, Rab11, in the activation of Rabin8. In addition, we will examine the phosphorylation and regulation of Rabin8 GEF activity by Extracellular signal-regulated kinases 1/2 (ERK1/2) in response epidermal growth factor (EGF) signaling. Finally, we will determine the role of the exocyst in SNARE-mediated membrane fusion. We take a multi-disciplinary approach combining biochemistry, biophysics, yeast genetics and microscopic imaging to address key questions in the field. These studies will not only help us elucidate the regulatory mechanisms of exocytosis at the molecular level, but also contribute to our understanding of a number of diseases such as diabetes and polycystic kidney diseases (PKD).